Will the Dragon faction have much in siege or will they just rely in the power of the Dragons themselves?

Dragons get both options, they can use their slave units to support their own siege weapons during castle assualt, of they can just fly the dragon in, burn the crap out of gate defenders, and open the gates that way

For the record - our gates work similar to Medieval Total War: there is a way to open them from inside the town, if you manage to kill all enemy units around.